As is known in the art, large mainframe computer systems and data servers sometimes require large capacity data storage systems. One type of data storage system is a magnetic disk storage system. Here a bank of disk drives and the computer systems and data servers are coupled together through an interface. The interface includes storage processors that operate in such a way that they are transparent to the computer. That is, data is stored in, and retrieved from, the bank of disk drives in such a way that the mainframe computer system or data server merely thinks it is operating with one mainframe memory. One type of data storage system is a RAID data storage system. A RAID data storage system includes two or more disk drives in combination for fault tolerance and performance.
As is also known in the art, it is sometimes desirable that the data storage capacity of the data storage system be expandable. More particularly, a customer may initially require a particular data storage capacity. As the customer's business expands, it would be desirable to corresponding expand the data storage capacity of the purchased storage system.
As is also known in the art, Small Computer Systems Interface (“SCSI”) is a set of American National Standards Institute (“ANSI”) standard electronic interface specification that allow, for example, computers to communicate with peripheral hardware. SCSI interface transports and commands are used to interconnect networks of storage devices with processing devices. For example, serial SCSI transport media and protocols such as Serial Attached SCSI (“SAS”) and Serial Advanced Technology Attachment (“SATA”) may be used in such networks. These applications are often referred to as storage networks. Those skilled in the art are familiar with SAS and SATA standards as well as other SCSI related specifications and standards.
One arrangement or expanding a data storage system is shown in FIG. 1. Here, a storage processor (SP) used in such system is shown coupled to a Disk Array Enclosure (DAE) having two sections of disk drives, each section here having twenty-four disk drives. A link controller card (LCC) is included here having a pair of SAS expanders. Each one of the SAS expanders is coupled to a corresponding one of the two disk array sections and is also to a corresponding on of a pair of SAS ports, as indicated. Thus, each one of the expanders is coupled to the storage processor (SP) through separate SAS expanders. It is also noted that the DAE includes a power supply section, indicators & sensor section, and a cooling section. These sections are controlled by both expanders, as indicated.
With this arrangement, because two expanders are used in the DAE the DAE does not appear as a single enclosure to the SP because there are two logical points of entry into the system and thus with the twenty-four disk drives per expander, each set of twenty-four drives appears in separate SAS domains. Further, using separate expanders results in shared enclosure functions such as power, cooling and indicators are managed from both expanders; therefore, the SP requires special knowledge that both expanders are actually in the same enclosure. Further, at the present time there is no single expander available that has enough ports for 48 drives plus expansion cables.
In addition, with this arrangement, there are signal Integrity problems that may limit SAS cable lengths or limit disk drive choices. More particularly, enclosure used in such an arrangement would be very large in order to accommodate forty-eight disk drives and SAS paths would be relatively long. i.e., the expanders must be placed to favor cable length or disk drive signal path length.